1. Field of the Invention
The invention relates generally to the field of sensors.
2. Description of the Related Art
Sorption-based microsensors are currently a leading candidate for low-power, compact chemical vapor detection for defense, homeland security, and environmental monitoring applications. Such a sensor combines a nonselective transducer with a chemoselective material that serves as a vapor concentrator resulting in a highly sensitive detector that responds selectively to a particular class of chemical vapor. An array of such sensors, each coated with a different chemoselective material, produces a response fingerprint that can detect and identify an unidentified analyte. Sorption-based sensors provide sensitive detection for vapors ranging from volatile organic compounds to semi-volatile chemical nerve agents, although low-vapor-pressure materials such as explosives are challenging because they do not produce a sufficiently high concentration of vapor.
The transducer elements for such sensor arrays need to be small, low power, and compatible with conventional microprocessing technology. Among the choices of transducers are chemicapacitors that detect changes in dielectric properties, microresonators that detect changes in mass and viscoelastic properties of the chemoselective coating during analyte exposure, and chemiresistors that monitor the resistance of a conductive polymer or polymer composite. Chemiresistors are simple to implement, but can suffer from instability of the conductive particle/polymer interface, slow response and insufficient sensitivity. Microresonators can suffer from instability in the resonator-coating interface and are higher power devices. Chemicapacitors are more stable but can suffer from slow response. As with chemiresistors, this slow response is a result of the time necessary to load and then remove the analyte from the relatively thick layers of chemoselective material (˜1 μm) that are typically used.
In a typical geometry for a commercial capacitance-based chemical sensor (Seacoast Sciences Inc.), the top electrode is a suspended grid of lithographically defined metal wires or a perforated membrane that allows chemical/biological analytes to pass between the wires into the active region of the capacitor. The sorbent layer between the top and bottom electrodes is a chemoselective material that selectively absorbs a class of chemical or biological analyte. Upon absorption of the analyte the dielectric properties of the chemoselective material are modified, which in turn produces a change in capacitance between the top grid and bottom planar electrodes. The capacitance, C, is measured by applying an AC voltage, VAC, to the device and detecting the AC current IAC=ωCVAC where ω is the AC frequency.
Such a transduction mechanism is highly stable; however, the analyte must diffuse laterally through the polymeric dielectric a distance that is comparable to the lithographic linewidth in order to produce a maximum signal. In order to maintain a low fabrication cost the minimum linewidth of the top capacitor plate is ˜1 μm. Consequently, this lateral diffusion, driven by a concentration gradient, results in a long response time (˜minutes) and limits the sensitivity.
The unique structural and electrical properties of single-walled carbon nanotubes (SWNTs) have inspired researchers to investigate and develop SWNT-based chemical and biological sensors. Initial work in this area has shown that the resistance of SWNTs changes in response to the exposure to certain molecules that undergo a charge transfer upon adsorption on the SWNT surface (Kong et al., “Nanotube Molecular Wires as Chemical Sensors”, Science, 87, 622 (2000). All reference publications and patents are incorporated herein by reference). Such SWNT-based chemiresistors have been used to detect both toxic industrial chemicals and a simulant for chemical nerve agents. In addition, by incorporating random networks of SWNTs as the active sensor element such sensors can be manufactured using conventional microfabrication techniques (Novak et al., “Nerve agent detection using networks of single-walled carbon nanotubes”, Appl. Phys. Lett., 83, 4026 (2003); Snow et al., “Random networks of carbon nanotubes as an electronic material”, Appl. Phys. Lett., 82, 2145 (2003)), circumventing the need for precise assembly. However, SWNT chemiresistors also exhibit several undesirable properties such as a high level of 1/f noise, slow recovery, and a susceptibility to contact effects.